Welding method and apparatus



Nov- 28, 94 J. H. COOPER ETAL WELDING METHOD AND APPARATUS Filed Nov. 5,1942 5 Sheets-Sheet l JOSEPH H. COOPER cum: E. SMITH f 77 film/w ao f 82/1| Nov. 28, 1944.

J. H. COOPER ETAL WELDING METHOD AND APPARATUS Filed NOV. 5, 1942 3Sheets-$heet 2 JOSEPH H. COOPER CLYDE E. SMITH Nov. 28, 1944. J. H.COOPER ET AL WELDING METHOD AND' APPARATUS s Sheets-Sheet 5 Filed Nov.-5, 1942 DESIRED FINAL WELDIING TEMPERATURE ANNEALING TEMP.

TIME

JOSEPH H. COOPER CLYDE E. SMITH Patented Nov. 28, 1944 2,363,719 WELDINGMETHOD AND APPARATUS Joseph H. Cooper, Warren, and Clyde E. Smith,

Port Homer, Ohio, .assignors to The Taylor- Winfield Corporation,Warren, Ohio, at corporation of Ohio Application November 5, 1942,Serial No. 464,602

28 Claims.

This invention relates to the artof welding metals and more particularlyto improved methods and devices for controlling the application of heatto the point or zone of weld.

While heretofore various systems have been devised and employedtocontrol the application of heat in electric resistance, are and flamewelding, such systems have utilized as their primary operational factorsthe furnishing of a predetermined quantity of heat or heating energy fora predetermined length of time or alternatively the furnishing of heator heating energy in a more or less uncontrolled manner until the finaldesired welding temperature is attained in the work. It is at onceapparent that these methods do not take into consideration the physicalresistance to heat transfer or in the case of electric resistancewelding, at least, the ohmic resistance and reactive impedance affectingthe rate of energy transfer as well as the physical resistance to heattransfer. Consequently in systems employing a timed heat input cycle theoptimum temperature in the work may or may not be attained dependingon-the presence and variations in a substantial number of normallyuncontrollable factors.

The above mentioned factors are afiected by the thickness and surfacecondition of the work, the area and condition of the welding electrodes,the welding pressure applied, the metallurgical analysis and characterof the metal of the work as well as the physical nature or form of thepieces (being Welded. Regarding the latter it isnow understood that theheat or current flow is localized at the desired point or area to adegree dependent on the existence of adjacent low resistance path areasor zones. For example, if the weld is to be made in the center of largeheavy sheets the localization of heating will be much less than if theweld be made at an edgeof the work, it being assumed that the samequantity of heat or energy is furnished in each case. Also in the caseof electric resistance welding the position of magnetic work materialsin the throat of the welding machine materially afiects the quantity ofenergy delivered to the work by reason of the variation in impedanceinthe welding load circuit.

The primary object of the invention is the provision of improved methodsand devices for controlling the application of heat in metal weldingoperations whereby the presence of and variations in the above mentionednormally uncontrollable factors does not and cannot aiiect the uniformquality attained in the completed welds. This object is accomplishedprimarily, in accordtermined interval of time.

ance with the present invention, by varying the rateof heat input inaccordance with the instantaneous rate of temperature rise at the pointof weld whereby the optimum final desired welding temperature is alwaysattained within a practicable period of time regardless of the effect ofthe specifically mentioned mitigating factors.

In welding certain metals, particularly hardenable steels which requirean annealing operation immediately following the welding operation, itis considered desirable that the effective rate of temperature rise inattaining the welding temperature be kept substantially constant at apredetermined value in order that the weld will have uniform and desiredphysical characteristics at the end of the complete operation. Also theattainment of the optimum welding temperature .within a practicableinterval of time is highly desirable in that the welding machine may bemaintained in operationat its most eflicient rate of production.

A further object of the invention, therefore, is the provision of animproved system for controlling the flow of heat or heating energy intothe work at the point of weld wherein the optimum welding temperature isalways attained within a predetermined time interval. This generalobject is accomplished, in accordance with the present invention, bycomparing a condition responsive to the temperature attained at thepoint of weld in any particular instant with a condition variableaccording to time and varying the rate of heat input in a manner tendingto maintain a balance between the said two conditions.

A further and more specific object of the invention'is the provision ofan improved method for welding hardenable steels which, as pointed outabove, require the application of an anneal-' ing cycle or operationfollowing the completion of the actual welding operation. The annealingmay be accomplished in difierent ways, depending on the final resultdesired, the physical characteristics of the steels, and theform of'themass of the work. For example, immediately upon the attainment of theoptimum welding temperature heat may be supplied at a lower rate forannealing purposes or the supply of heat may be discontinued entirely atthis time until the temperature at the weld has dropped to a desiredannealing temperature after which heat or heating energy is controllablysupplied to maintain this desired annealing temperature for a prede-Regardless of the particular manner in which the annealing is effectedsubstantially improved results are at- The discharge devices l3 and Mare preferably of the mercury arc type employing-an ignition or startingelectrode in addition to-an anode and a ate of heat. or heating energyinput, and a conr01 therefor including means to actually sense hetemperature of the work at the point of weld and further including atime reference element vhereby all heating energy furnished is always:uflicient to effect a predetermined rate of temmerature rise. In a morespecific aspect the sysem may include a further timing element to proidean annealing period of predetermined dura; ion.

The principles outlined above as well as the tated objects andadvantages of the invention vill be better understood upon aconsideration of he following detailed specification and the ac-:ompanying drawings, wherein there is specificaly disclosed certainpractical embodiments of the nvention. Further objects and advantageswill lecome apparent uponta better understanding of he broad andspecific features of the invention. .he scope and extent of theinvention itself,however, both as to method and apparatus, are deineatedin the appended claims.

In the drawings:

Figure 1, consisting of parts IA and IB, is a schematic diagram of awelding current con irol system embodying the principles of theinention;

Figure 2 is a modification of a portion of the yst'ems of Figure 1; and

Figure 3 is a temperature-time graph of a comlined welding and annealingoperation which may be followed while employing the main prin-' rgysource or to any particular method of apdying the energy to the work.The temperature f the work at the point or zone of weld may be measuredby any suitable device-a thermocouple r photocell responsive to infraredradiations, for xample-and various forms of intermediate conrols may beemployed.

In the preferred and illustrated embodiment f the invention,-however, apair of electronic or pace discharge devices l3 and I4, connected inaverse parallel, are provided to control the flow f current from analternating current source S hrough the primary winding of the weldingransformer III. In series with the source and rimary a capacitor Il maybe inserted to correct he power factor of ,the welding load if desired..he secondary winding of transformer I is con- .ected to the electrodesl5 and I6 of a resistance elding machine and incorporated or built intohe electrode It, in any suitable manner, is a hermo-couple I! thefunction of which is to ieasure the temperature of the work at the pointr zone of weld.

mercury pool cathode. In accordance with usual practice the ignitionelectrodes are energized in controlled phase relationwith the linevoltage by means of secondary or control discharge device's I9 and I8,the plate circuits of which are arranged to be connected with theignition electrodes of the respective discharge devices I: and I4.Discharge devices I8 and I9 are preferably of the grid controlled gasfilled type known commercially as Thyratrons. In the manner to behereinafter more fully explained conduction in the Thyratrons l8 and I9and consequently in the valves l3 and I4 is controlled by operation ofthe relay devices 20 and 2| which are also preferably of the gas filledelectronic discharge type. As is well understood in the art the lengthof the conduction period in the devices 20 and 2| in each cycle of .thealternating line current, is a function of .the resultant of theimpressed direct and alternating components of the grid biasing voltage,taking into account the phase relation of the alternating component. Inaccordance with the preferred and illustrated embodiment of theinvention these variable factors are utilized and controlled to effectthe desired method of to the welding load is obtained by shifting thephase of the alternating component'of the grid bias impressed on thespace discharge devices 20 and 2| with respect to their plate voltagesand this is accomplished by employing a phase shifting device 22 whichis connected to the supply line Ll, L2 through the transformer 23.Conductors 24, 25 and 26 lead from the phase shifting device 22 and thedevice is so adjusted that the alternations in the circuit 24, 25 iscloser'in phase with the alternations in the principal source S than arethe alternations in the circuit 24, 26. Common conductor 24 is connectedto an end terminal of the primary winding of each of two transformers T4and T5. The other terminals of primary windings of transformers T4 andT5 are arranged'to be alternately connected to conductors 25 and 26 bymeans of the conductor 21 and thecontactors 28 and 29 of the relay 30,the function and operationof which will be explained below. Thesecondaries of transformers T4 and T5, in series with a controlledsource of direct current biasing potential to be hereinafter more fullyexplained, furnish the altemating' component of the grid biasingpotential for the discharge devices 2i and 20, respectively, through theconductors 3| and 32, respectively. Thus it should be apparent that,assuming a constant direct current component of grid biasing potential,the period of conduction dur- The direct current component of the gridbias- -ing voltage for the discharge devices 20 and 2| varies in valuein accordance with the deviation of a predetermined fixed relation ofthe temperature attained at the point of weld-with respect 7 to time asmeasured from the initiation of the welding cycle. Thus, a control basedon the rate conductor 1 I.

of temperature rise is eflected. In operation, this control is eifectiveuntil the final desired welding temperature is reached (time Tl) afterwhich the weld may be allowed to cool down to a predetermined desiredtemperature, which temperature is maintained for a predetermined desiredlength of time, all as indicated in Figure 3. The circuits resulting inthe constant rate of temperature rise during the period Til-Tl will nowbe described.

The leads 33 and 35 of the thermo-couple l1 are connected to the endterminals of potentiometer 35, one end of which is connected to groundthrough a battery 36. The tap 31 of the potentiometer 35 is connected tothe grid of a vacuum tube amplifier 38 through conductor 39 and it 4|,the source of current being the transformer 451 having its primaryconnected to' the line conductors LI and L2. The voltage developedacross the load resistor 42 in the plate circuit of the tube 38 isimpressed on the grid of a second vacuum tube amplifier 43. A rectifiedand well filtered plate current supply for the tube 43 is furnished bythe full wave rectifier 44 ,and the pi-section filter 45, the initialsource being the transformer T which has its primary connected totheline conductors Ll, L2.

A full wave rectifier 46 acting through the filter 41 furnishes a directcurrent biasing potential source across the conductors 48 and 49, whichpotential upon the closing of contactor 56 is impressed across theresistor contactor 50 may be part of arelay 52, the operating coil 53 ofwhich is arranged to be energized from the control current source lineunder the control of a switch 54 which, in accordance with usualpractice, may be pressure actuated to close upon the attainment of apredetermined pressure between the welding electrodes and the work.Another source of direct current biasing potential consisting of arectifier 55, condenser 56 and load resistor 51 is provided and thenormal direct current component of grid biasing potential for thedischarge devices and 2| upon the closing of contactor 50 is provided bythe following circuit: Cathodes of devices 20 and 2|, conductor 58,portion 53 of potentiometer 5|, conductor 60, resistance 6|, conductor62', resistance 51, conductor 63, resistance 64, resistance 65,conductor 66, and through the secondary windings of the transformers T4and T5 and the conductors 3| and 32 to the control grids of thedischarge devices 20 and 2|. Resistancev 6| together with a secondresistance 61 which is in series therewith is in the plate circuit of agas filled discharge device 58 as is also the conductors 46 and 48.Normally, the device 68 is held non-conductive by the negative grid biasobtained from the negative end of resistor 51 and impressed throughconductor 68, the secondary winding of transformer 10, and

Transformer 16 is of the impulse type, its primary being connected tothe control current source line Ll, L2 through the variable resistanceshould be apparent that the plate. current in the W 12 which is soadjusted that the peak of the output of the transformer will occur nearzero voltage of thecontrol, current source. The impulse furnished by thetransformer 10 to the conductor 1| in apositive direction overcomes thenegative bias on the thyratron 88, causing it to conduct. There will nowbe developed a voltage across the series resistances 6| and 61 which isequal to the voltage impressed by the rectifier 46, 41 across theresistor 5| less the internal voltage drop across the discharge device68.

Assuming there is 450 volts across the resistor 5| the drop across theresistances 5| and 61 will be 435 volts if the drop in tube 68 is 15volts. If now rectifier 55 develops volts across resistor 51 and theresistance 59 be adjusted to provide a drop of 200 volts betweenconductors 58 and 6|] an adjustment of resistance 6| to provide a dropof 280 volts .between the conductors 62 and 60 will result in zerovoltage between the conductors 58 and 63. Upon conduction of tube 68 thealternating component of grid biasing potential I furnished by thetransformers T4 and T5 is operative to cause conduction in the devices20 and .2| according to the phase relation of their gridcathode voltagesas determined by the phase shifting device 22.

Normal negative grid biasing voltage for the space discharge device I8is furnished by a rectifier 12 having load resistor 13, the biasingcircuit being traceable from the cathode, resistor 13, conductor 14, theprimary of transformer T1, and the conductor 15 to the grid. The deviceI8 is thus held non-conductive until the discharge de vice 20 isrendered conductiveand furnishes a positive bias wave to the primary oftransformer T1. Upon conduction or "firing of the device l8 the ignitionelectrode 16 of the valve I4 is energized through the circuit lineconductor WLI, conductor 11, contactor 18 of relay 52, conductor 19,electrode 16, and mercury pool cathode ofpositive biasing wave isimpressed on the grid of device I 9 through transformer T5 and uponoperation or firing" of the device 9 the ignition electrode of the valvel3 will be energized through the circuit WL2, conductor 86, contactor 81of the relay 52, conductor 88, electrode 85, mercury pool cathode 3, andline conductor WLI, causing valve l3 to conduct.

It will be noted that the equal series resistances 64 and 65 are inseries with the source of the direct current component of the gridbiasing volta e for the control devices .20 and 2| and it will beapparent that if the end terminals C and D of these resistances are atequal potential the balance of voltage in the conductors 58 and 62 (or56) will not be disturbed. If however, a voltage drop should developfrom D to C the direct current component of the biasing potentialimpressed on the grids of the control devices 20 and 2| will become morepositive thus causing these devices to conduct earlier in the voltagehalf cycles thereby increasing the welding current supplied to the work.Conversely a voltage drop in the opposite direction will cause thedevices 20 and 2| to conduct later in the voltage half cycles therebydiminishing the current passed through the work. In accordance with thePreferred and i1lustrated embodiment of the invention the rate oftemperature rise iswontrolled by balancing the rate of potential rise atthe point D against the rate of potential rise at the point C. It willbe observed that the resistor 65 is in the plate circuit of theamplifier 43 through conductor 89, conductor 90, con-tactor ill of relay30, and conductor 92. As explained above, the plate current in amplifier.43 varies directly with the temperature of thermocouple I1 and itshould be apparent that as the temperature rises'the potential at pointD becomes more negative. Means nowto be dee scribed is provided to givea predetermined rate ofpotential drop at the point C.

Connected with conductor 48 is a conductor 93 and across thisconductor-93 and the conductor 60 (which is a continuation of conductor49) is a potentiometer 94, the adjustable tap of which is connectedthrough conductor 95, contactor 96 of relay 52, conductor 91; andadjustable resistance 98, to the positive terminal of a capacitor 99.The negative end of capacitor 99 is connected to conductor 60 throughconductor I00. Thus the rate I of charge of the capacitor 99 upon theclosing of contactor 98 is determined by the setting of thepotentiometers 94 and 98. The potential attained by the postive terminalof capacitor 99 is impressed on the grid of an amplifier I0! throughconductors I02 and I03. As shown, the resistance 64 is in the platecircuit of the amplifier WI and it should be apparent that the point Cwill become progressively more negative as the grid potential becomesmore positive at a rate determined by the rate of charging of thecapacitor 99. If the thus determined rate of potentialdecrease atthepoint C leads the rate of potential decrease at the point D'apotential drop will appear across D to C which drop is subtractive tothe "negative biasing potential impressed on the control grids of thecontrol devices 20 and 2I thereby increasing the current supplied .thewelding, load. Conversely a lag in potential decrease of the point Cwith respect to the point D will decrease the current supplied thewelding load. It will be understood that the capacitor 99 will begin tocharge immediately upon energization of coil 53 of relay energizing thatcircuit above outlined which includes the coil I01. 7

When relay 30 is actuated by energization of coil I01 one of itscontactors II8 closes to short out the resistors 64 and 65 from thecircuit which furnishes, the direct current complement of grid biasingvoltage to the control devices 20. and 2I. At the same time contactor 9Iopens to relieve the direct current voltage at the leads 90'and 92.Conductor is disconnected from and conductor 26 is connected to theprimary windings of the transformersT4 and T5 so that during this perioda lesser value of current as deter- .mined by the phase angle of thevoltage wave in conductor 26 is passed through the work. This value isfixed by preadjustment of the phase shifting device 22.

Having its negative terminal connected to the conductor 60 is a timingcapacitor I20, the positive terminal of which is connected with thecathode of the gas discharge device 68 through an adjustable resistanceI2 I, conductor I22, contactor I23 of relay 30, and conductor I24. Thus"immediately upon the energization of relay 30 the voltage begins tobuild up on the capacitor I20 at a rate dependent on the setting ofthepotentiometer l2I. The positive terminal of the capacitor I20 isconnected to the control grid of device I25 will begin to conduct.

a gas filled discharge device I25 and upon the capacitor attaining apredetermined charge, the Device I25 shunts that portion of the resistorM which is exclusive of the portion 59 and upon conduction it places avery high positive potential on the conductor 58 which in turn forcesthe grid bias of the control devices-20 and 2I highly negative thusenabling the grids to regain control and stop the flow of all weldingcurrent.

The above described operation of the timing. I

capacitor I20 provides for an annealing period of predetermined durationand of predetermined current strength (as determined by the setting ofthe phase shifting device 22) immediately upon the attainment of thefinal desired welding tem- I terval under the control of the temperaturere- Coil I01 of the relay 30 is arranged to be energized from asecondary winding of transformer 23 through conductor I08, acontrolledgas discharge device I09, conductor IIO, contactor III pearing at theadjusted tap of a potentiometer v IIB which in turn is connected betweenground and the lead 93 of the thermo-couple I1 When the temperature atthe work or thermo-couple reaches the final desired welding temperatureat time TI as determined by the setting of potentiometer I16 the platecurrent in the discharge device II5 will go tozero or cut-om This actionremoves the negative bias from the grid of device I09 causing the latterto conduct thereby sponsive device to maintain a. constant temperatureduring the annealing interval. Also, relay 30 may be of a time delaytype so that upon the attainment of the final desired weldingtemperature and the controlled interruption of the flow of weldingcurrent a measured time interval (TL-T2) may elapse before the relay ispulled in and current flow (for annealing purposes) is rte-established.The absence of current flow during this interval provides a coolingofiperiod to enable the work to reach'its proper annealing temperature.

Upon completion of the heat cycle of the welding or welding andannealing operation switch 54 is opened thereby de-energizing coil 53.The

opening of relay 52 also de-energi'zes coil I01 of cally disclosedwithout detracting from the-essential and desired mode of operation. Forexample, instead of controlling the reference rate of change ofconduction in the amplifier IIII in accordance with the rate of chargeof a timing capacitor the same may be accomplished in the manner shownin Figure 2. A tapped voltage dividing resistor I30 is connected inseries with the variable resistance I3I and a contactor I32 of the relay52 across the conductors 60 and 93. An arm I 33 adapted'to be driven ata predetermined rate of speed by a controlled speed motor I34 isarranged to connect the grid of the amplifier IIII with the taps of theresistor I30 in sequence.

From the foregoing description of the illustrated embodiments of theinvention it is apparcut that upon closureof switch 54 and energize.-tion of solenoid 53 of relay 52, contactors 18 and 81 connect the platecircuits of the firing tubes I8 and I9, respectively, with the ignitionelectrodes of the valves I4 and I3 while the contactor 50 .eonnects theplate circuit of the gas discharge device 68 (including the seriesresistances SI and 61) as well as the resistor across the output.

of v the rectifier 46, 41. As the voltage of the source next passesthrough zero in a positive di rection the impulse from transformer Illstarts conduction in device 68 thereby building up po tential acrossresistance BI which, as outlined above, removes the negative biasingpotential impressed on the grids of the control devices and 2|. Thiallows the alternating component of the biasing potential from thetransformer T4 and T5 to cause conduction in the devices 20 and 2|according to the phase relation of their grid cathode voltages asdetermined by the setting of the phase shifting device 22.

If the quantity of energy passed as a result of the above alternatingcurrent biasing component is sumcient to effect the desired rate oftemperature increase, terminals C and D of the resistors B4 and 65remain at equal potential thus not effecting the period of conduction inthe valves I3 and I4 during'the half cycles of the alternating currentsource. However should the condition not remain stable the resistors .64and 65 operate, as explained above, to superimpose either a negative ora positive increment of continuous biasing potential on the gridcircuits of the devices 20 and 2| thus decreasing or increasing thelength of the conduction periods.

The control illustrated operates in the above manner, until the finaldesiredwelding temperature is reached at which time the output ofamplifier H5 is suificient to start conduction in the gas dischargedevice I 09 thereby pulling in relay 30. Contactor II 8 now removesresistors 8 and 65 from the grid biasing circuit for thecontrol devices20 and 2| and contactors 28 and 29 operate to shift the phase of thealternating current biasing source to decrease the length of the periodsof conduction for annealing purposes.

This annealing time is controlled by the rate-of charging of thecapacitor I 20 and upon a.predetermined potential being reached the gasdischarge device I .becomes conductive thereby shunting that portion ofresistor 5| which is exelusive of portion 59 and making'the potential inconductor 58 highly positive. This forces the grid bias of devices 20and 2| highly negative and stops the flow of welding current. Theopening 5 2 de-energizes coil I 01 of relay 30 and consequently all theparts are thereupon restored to their initial condition of readinesspreparatory to the initiation of the next succeeding weldingoperation. I

It should now be apparent that we have provided an improved weldingsystem which accomplishes, in a practical manner, the objects initiallyset out. By basing the welding current or heat control essentially onthe rate of temperature'increase of the work at the point of weld manynormally uncontrollable factors afiecting the final welding results andnot heretofore compensated for are rendered ineffectual. In addition tothe factors mentioned above, is the var-iation, in resistance welding,of current intensity at the point of weld due to the existence of shuntpaths provided by previously welded spots. If the spots are closetogether the current flow will be more distributed thus decreasing thecurrent intensity and rate of heating at thespot being welded. It shouldbe observed, however, that the system of the invention, whileautomatically providing compensation for the various factors stated,nevertheless enables the successive welds to be accomplished inessentially constant and equal periods of time thus enabling th machineto operate at its most efflcient rate of production. The apparatusherein described is susceptible of wide modification within the scopeand spirit of the invention, the apparatus being disclosed in detailmerely for the purpose of illustrating the practical applicability ofthe invention, and reference should accordingly be had to the appendedclaims and the prior art in appraising the extent of the invention.

We claim: 1. The method of resistance welding which consists of applyingwelding electrodes to the work to be welded, supplying welding currentto the electrodes, determining the temperature attained in the weldingzone of the work by the flow of welding current therethrough, andcontrolling the strength of the welding current during the welding cyclein accordance with instantaneous values of said determined temperaturewhereby a predetermined rate of temperature rise in the welding zone ofthe work may be effected.

2. The method of welding hardenable steels which consists of applyingwelding electrodes to the work to be welded, supplying electric weldingcurrent of variable strength to the electrodes in accordance withinstantaneous values of the temperature attained in the work during thewelding cycle at the point of weld whereby a predetermined rate .oftemperature rise in the welding zone of the work is effected,interrupting said control when the final desired welding temperature isreached, andthereafter passing through the electrodes and work a currentof less value than the current previously supplied to provide for theannealing of the metal in and adjacent to the zone of weld.

3. The method of Welding hardenable steels which consists of applyingelectric current and pressure to the work to be welded, controlling thestrength of the current in accordance with of switch 54 de-energizescoil 53 of relay 52 the instantaneous values of the temperature attainedin the work during the welding cycle at the point of weld whereby apredetermined rate of temperature rise in th welding zone of the work iseffected, interrupting said control when the final desired weldingtemperature is reached, and thereafter passing through said zone and fora substantial length oftime a current of less value than the. currentpreviously supplied to provide for the annealing of the metal inandadiacent to the zone of weld.

4. The'method of welding hardenable steels which consists ofapplyingelectric current and ouslylsupplied to provide for the annealingof" the metal in and adjacent to the zone of weld.

5. The method of electric resistance welding which consists of applyingelectric current and pressure to the work at the point of weld, andduring the welding cycle controlling the strength of current inaccordance with instantaneous values oi the temperature attained in thework at the point of weld. v 6. The method of electricresistance-welding whichconsists of appl electric current and pressureto the work at the point of weld, determining instantaneous values ofthetemperature attained at the point of weld, and controlling the strengthof the current in accordance with the rate of temperature increase asdetermined from said values whereby a predetermined rat oi temperatureincrease may be enected.

7. The method of, electric resistance welding which consists of applyingelectric current and pressure to the work at the point of weld,determining instantaneous values of the temperature attained at thepoint of weld, controlling the strength of the current in accordancewith the rate of temperature increase as determined from said valueswhereby a predetermined rate pressure to-the work at the point of weld,varying the strength oi the current during the welding cycle inaccordance with the rate of temperature increase in the work at thepoint of weld whereby a predetermined rate of temperature increase iseflected until a final desired welding temperature is reached, theninterrupting the flow of current until the metal atthe point of weld hascooled to a proper annealing temperature, and thereafter applyingcurrent of proper strength to maintain said annealing temperature.

9:. In a resistance welding system having a welding load circuit with animpedance factor, the method of automatically compensating for normallyuncontrollable variations in said factor which, consists of varyingduring the welding cycle the strength of the current furnished saidcircuit in accordance with the rate of. the'temperature rise in the workat the point of weld.

3 having a welding load circuit with an impedance factor, the methodofiautomatically compensating for my uncontrollable variations in saidfactor which consists of furnishing controlled portions of the energy ineach energy source impulse 1 to the welding load circuit, and varyingsaid portions in accordance with the rate of temperature rise in thework at the point of weld.

11. A welding system having a welding load circuit adapted to beconnected to an alternatin current source, a pair of oppositelyconnected electric valves interposed between said source and circuit,each of said valves having a control elec-' trode and a plurality ofprincipal electrodes, means to conduct energizing current to saidcontrol electrodes, means to control the flow of current to said controlelectrodes in propertimed sequence to condition said valves forconduction during at least a portion of each half cycle of the sourcecurrent comprising a pair of space discharge devices each having acontrol electrode, and a source of blas ing potential for said lastmentioned control electrodes comprising means for varying the biasingpotential in response to the rate of temperature increase in the metal.being welded.

12. A welding system having a'welding load circ-ult connected to analternating current source through space discharge means operative toconduct current to said circuit in half cycle incre-" merits, and meansresponsive to the rate of temperature increase in the metal being weldedto control the time in the half cycles at which said discharge meansbecomes conductive.

13. A welding system having a welding load circuit connected to analternating current source through space discharge means operative tocon- 1 ductive, said control means comprising a space discharge devicehaving an alternating anodecathode potential in phase with said source,said discharge device having a control electrode, and means responsiveto the temperature attained in the metal being welded in control of thepotential furnished said control electrode whereby the quantity ofenergy conducted to said circuit during each half cycle of said sourceis varied in ac-' cordance with said temperature.

14. A welding system comprising a source of electric welding energy, awelding load, modulat-' ing means to control the strength of weldingcurrent furnished said load from said source, means to determine thetemperature attained in the work at the point of weld, means operativeto provide a reference factor variable according to time, and means tocontrol the operation of said modulating means in accordance with therelation of said temperature to said factor during each,

succeeding increment of time.

15. A welding system comprising a. source of electric welding energy, awelding load, modulating means to control the strength of weldingcurrent furnished said load from said source, means responsive to thetemperature attained in the work at the point of weld, means including atiming capacitor to provide a reference factor variable according totime, and means responsive to 4 variation in difierential between-saidfactor and temperature to control said modulating means.

16. Apparatus according to claim 14 further including means to rendersaid last named means to control inoperative upon a predeterminedtemperature being reached.

17. Apparatus according to claim 14 further in and means operative aftersaid predetermined temperature is reached to render said modulatingmeans conductive for a predetermined interval of time as determined bysaid second timing device.

18. A welding system comprising a source of electric welding energy, a.welding load, modulating means to control the strength of weldingcurrent furnished said load from said source, an electronic dischargedevice having an anode, a cathode and a control element in control ofthe operation of said modulating means, component direct and alternatingcurrent sources of biasing potential for said control element, means toapply said direct current component during a'portion of a completewelding cycle and in accordance with the instantaneous temperaturesattained at the point of weld,'and means to apply said alternatingcurrent component during a latter portion of said complete weldingcycle.

19. In a resistance welding system, the method of automaticallycompensating for normally uncontrollable variations in the intensity of,the flow of heating current at the point of weld as caused by thepresence in the work of previously ,made welds providing shunt paths,for example, which consists of varying during the welding cycle thestrength of the current furnished the welding circuit in accordance withchanges in the instantaneous values of the temperature actually attainedin the work at the point of weld at any particular time during thewelding cycle.

20. Welding'control apparatus comprising in combination means tomodulate the flow of welding current to the welding load, and meanscomprising means responsive to instantaneous values of the temperatureattained'in the welding zone during the welding and also comprisingmeans providing a time reference factor to control said modulating meanswhereby a predetermined timewelding temperature relation is maintainedthroughout at least a substantial portion of the welding cycle.

21. The method of electric resistance welding which consists of applyingwelding pressure to the work at the point of weld, supplying heatingcurrent to the zone of weld, and varying the intensity of said currentin relation to time and to instantaneous values of the temperatureattained in said welding zone during the welding cycle to maintain apredetermined time-welding temperature relationthroughout at least asubstantial portion of the welding cycle.

22. The method of welding hardenable steels which consists of applyingwelding pressure to the work at the point of weld, supplying heatingcurrent to the weld, modulating said current in accordance withinstantaneous values of time and in accordance with the temperatureattained in the zone of weld during the welding cycle whereby apredetermined time-temperature relation is attained throughout thewelding cycle, interrupting said heating current upon the welding zoneattaining a predetermined maximum temperature, and thereafter applyingan annealing current to the zone of weld.

23. A welding system comprising a weldingload circuit, a source ofwelding energy, means to modulate the flow of energy from said source tosaid circuit, grid controlled space discharge apparatus. in control ofsaid modulating means,

and means to control the grid bias on said apparatus comprising meansestablishing a potential variable according totime and meansestablishing an opposing potential variable in accordance with theinstantaneous welding temperature attained.

24. A welding system comprising a welding load circuit, a source ofwelding energy, means to modulate the flow of energy from said source tosaid circuit, grid controlled space discharge apparatus in control ofsaid modulating means,

and means to control the grid bias on said apparatus comprising meansestablishing a potential variable according to time and meansestablishing an opposing potential variable in accordance with a weldingcondition, said source being aperiodic and further including means toimpress on said grid biasing potential a periodic component of the samefrequency as said source, and means to vary the phase relation of saidcomponent with respect to said source.

25. A welding system comprising a welding load circuit, a source ofwelding energy, means to modulate the flow of energy from said source tosaid circuit, grid controlled space discharge apparatus in control ofsaid modulating means, means to control the grid bias on said apparatuscomprisingmeans establishing a potential vari- ;able according to timeand means establishing an opposing potential variable in accordance withthe temperature attained at the point of weld, and means to interruptsaid flow of energy upon said temperature reaching a predeterminedmaximum value.

26. A system according to claim 25 further characterized in that saidsource is aperiodic and further including means to impress on said gridbiasing potential a periodic component of the same frequency as saidsource, and means to vary the phase relation of said component withrespect to said source.

27. A welding system comprising a welding load circuit, a periodicsource of welding energy, means 7 u W to modulate the flow of energyfrom said source to said circuit, grid controlled space dischargeapparatus in control of said modulating means, means to control the gridbias on said apparatus comprising means establishing a potentialvariable according to time and means establishing an opposing potentialvariable in accordance with the temperature attained at the point ofweld, means operable upon said temperature reaching a predeterminedvalue to render ineffectual both said establishing means, and meansoperable after said predetermined temperature is attained to impress onsaid apparatus a biasing wave of the same frequency as said source andhaving controlled phase relation with respect thereto.

28. A welding system comprising a welding load circuit, a source ofwelding energy, means establishing a time factor, means responsive tothe temperature attained at the point of weld, means including saidestablishing means and said responsive means to modulate the flow ofenergy from said source to said circuit to maintain a predeterminedtime-temperature relation, means to control said modulating means toprovide for the flow of annealing current upon the com-

